Borehole stability analysis accounting for anisotropies in drilling to weak bedding planes

Borehole instabilities pose significant challenges to drilling and completion operations, particularly in regions with weak bedding planes and pre-existing fractures where formations have strong anisotropies. The bedding planes, rock anisotropy, and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling. This improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions. Laboratory test data of rock compressive strengths are analyzed, and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities. Time-dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time. The slip failure gradient in the weak planes is derived, which can be used to model wellbore sliding/shear failure in the planes of weakness. The mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model. (C) 2012 Elsevier Ltd. All rights reserved.